The present invention relates to a color television receiver having an automatic cathode cutoff-voltage adjusting circuit for a color picture tube.
Color television receivers demodulate received video signals into video signals of three primary colors, red, green, and blue and supply the R, G, and B video signals respectively to the cathodes of a color picture tube to control electron beams from the cathodes which bombard phosphors of different colors on a fluorescent screen to enable the phosphors to emit proper amounts of red, green, and blue light, so that a colored picture image will be reproduced which has been transmitted from a transmitting station. A desired shade of color, including white, can thus be reproduced by additive color mixture of the three primary colors, red, green, and blue. When it is desired to reproduce white on the fluorescent screen, the image which should look white is tinted and all other colors on the screen become diferent from those which should appear on the screen if the three primary colors are mixed at a ratio which is different from that for forming white. Under these conditions, the quality of a reproduced color image is degraded.
To eliminate the foregoing problem, it is necessary to enable the three electron guns to operate under constant conditions at all times. However, the electron guns have different cathode cutoff voltages mainly because of irregularity in manufacture and besides, these voltages tend to vary with time while in use, resulting in different mixture ratios for the three colors. There has been proposed in, for example, Japanese Preliminary Patent Publication No. 44435/79 an automatic cutoff adjusting circuit for varying the voltages of video signals supplied to the cathodes of a color picture tube in response to changes in the cathode cutoff voltages. The proposed automatic cutoff adjusting circuit, however, is disadvantageous in that the color mixture ratios are liable to change due to leakage currents flowing between the cathodes and the heaters therefor.